Density functional theory investigation into the interaction of deep eutectic solvents with amino acids

Abstract

Processing of amino acids (AAs) is a very cost-intensive process. Recently, Li et al. used supported liquid membranes (SLMs) based on deep eutectic solvents (DESs) for amino-acid extraction and found that [choline chloride][p-Toluenesulfonic acid] ([ChCl][p-TSA])) DES was efficiently separating Tryptophan (Trp(•)) amino acid. Inspired by the study, here we investigate the interaction of [ChCl][p-TSA] DES with three forms of amino acids, including neutral(•), cationic(+), and anionic(-) forms using density functional theory (DFT) method at the M06-2X/6–311++G(d,p) level of theory. Our results indicate that the interaction of AAs(•/+/-) with [ChCl][p-TSA] DES is governed by the hydrogen bonding interactions, which is consistent with the findings of 1H NMR spectra by Li et al. Thermochemistry calculations indicate that the formation of [ChCl][p-TSA]…AAs(•/+/-) complexes is an exothermic and favorable reaction and proceeds spontaneously. Among the three forms of AAs(•/+/-), cationic forms (AAs(+)) exhibited the most tendency to interact with the [ChCl][p-TSA] DES. The binding energy (ΔEb) calculations show that Trp(•) has the strongest interaction with [ChCl][p-TSA] DES among the AAs(•), in agreement with the experimental results reported in the literature. The Gly(+) and Val(-) also have the highest ΔEb value with [ChCl][p-TSA] DES. A comparison between the calculated free energies and the extraction efficiency (%) reported by Li et al. for extraction of AAs(•) by [ChCl][p-TSA] DES indicates that the higher the free energy value, the higher is the extraction efficiency (%) of neutral AAs(•). Furthermore, the natural bond orbital (NBO) analysis, atoms in molecules (AIM) theory, and noncovalent interaction (NCI) plots were performed to determine the nature and strength of hydrogen bonding interactions between the [ChCl][p-TSA] DES and AAs(•/+/-). These analyses indicate that the O-H..O, O-H…[Cl]−, and N-H…[Cl]− interactions in the complexes are stronger than the N-H…O and C-H…O interactions. Energy decomposition analysis (EDA) method revealed that the charged AAs(+/-) have a greater tendency to interact with [ChCl][p-TSA] DES due to the presence of positive and negative charges on the AAs(+/-), which leads to increased strength in the electrostatic interactions between the AAs(+/-) and [ChCl][p-TSA] DES.